If you treat a Chao very well with a specific character for a while and didn't attack it, then you could check how happy they are by whistling. If they notice and run toward your character, then they are definitely happy enough to evolve into a Chaos Chao.

Keep in mind that whistling is only reliable if the Chao also hasn't had any negative interactions with any other characters. For example, even if my Chao has received plenty of affection from Sonic, if the Chao was repeatedly hurt by Knuckles, the Chao will still respond to Sonic's whistle, but still have lower happiness because of Knuckles.

chaoadventures wrote:Yeah, they still have actions to show how happy they are, including the heart, exclamation mark, and other things, they normally get above their head.

Their face doesn't change at all unless they're sleeping though.

Chaos Chao also blink and their eyes are half-open if you wake them up from their sleep. I've repeatedly done that by accident with Hero Chaos Chao because they sometimes re-spawn in a pool and I forget if they're sleeping, so when I pick the wrong one up, it looks cranky.